1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a fabrication method therefor, the solid electrolytic capacitor including a metal electrode used as an anode, and a dielectric layer of an oxide of the metal of the metal electrode and formed on a surface thereof. More particularly, the invention relates to a solid electrolytic capacitor featuring a decreased equivalent series resistance in high frequency regions.
2. Description of the Related Art
Recently, CPUs for use in personal computers have grown more powerful, bringing about a demand for a capacitor featuring excellent high-frequency characteristics with a low equivalent series resistance in high frequency regions as well as a great capacitance.
Capacitors employing a film, mica, ceramics or the like are known as the capacitor having the good high-frequency characteristics. Unfortunately, such capacitors generally have small capacitances and hence, these capacitors need be increased in size in order to achieve high capacitances. This also results in an increased cost.
More recently, electrolytic capacitors having high capacitances have been developed. The electrolytic capacitors fall into two types which include a capacitor employing a liquid electrolyte and that employing a solid electrolyte.
In the case of the electrolytic capacitor employing the liquid electrolyte, which is ion conductive, there is a problem that the equivalent series resistance is great in high frequency regions. Therefore, in an application requiring good high-frequency characteristics, the solid electrolytic capacitors employing the solid electrolyte are commonly used.
As disclosed in JP-A-63-173313, such a solid electrolytic capacitor is fabricated by anodizing aluminum or tantalum used as an anode member thereby forming an oxide of such a metal which defines a dielectric layer on the anode surface; and overlaying a layer of conductive polymer, such as polypyrrole or polythiophene, on the dielectric layer by chemical polymerization or electrolytic polymerization.
However, the conductive polymer, such as polypyrrole or polythiophene, has conductivity as small as a semiconductor. For instance, polypyrrole has a conductivity of 102 S/cm and hence, satisfactory high-frequency characteristics are not obtained because of the increased equivalent series resistance in the high frequency regions.
It is an object of the invention to provide a solid electrolytic capacitor comprising a metal electrode used as an anode, and a dielectric layer of an oxide of the metal formed on a surface of the metal electrode, the capacitor achieving improved high-frequency characteristics with decreased equivalent series resistance in high frequency regions.
According to the invention, a solid electrolytic capacitor comprises: a metal electrode employing a metal; a dielectric layer comprising an oxide of the metal and formed on a surface of the metal electrode; and a carbon material layer overlaid on the dielectric layer.
Where the carbon material layer is overlaid on the dielectric layer of the metal oxide as suggested by the solid electrolytic capacitor of the invention, the electric charge transfer is increased in comparison with a case where a conductive polymer, such as polypyrrole or polythiophene, is used. The increased electric charge transfer leads to the reduction of equivalent series resistance in high frequency regions and hence, the solid electrolytic capacitor featuring excellent high-frequency characteristics is provided.
In a mode, the metal electrode employing the metal may be anodized to form the dielectric layer of the metal oxide on the surface thereof.
Examples of a metal used in the anode include valve metals such as aluminum, tantalum, niobium and titanium, and alloys thereof. Above all, aluminum is less costly and has stable dielectric characteristics.
It is preferred that the carbon material layer on the dielectric layer of the metal oxide is deposited by electrophoresis using the metal electrode formed with the dielectric layer as the anode. The reason is as follows. The dielectric layer of the metal oxide is fragile and contains therein micropores. When the carbon material layer is overlaid on the dielectric layer, a solution containing the carbon material penetrates into the micropores of the dielectric layer so as to swell the dielectric layer, which will be cracked or separated from the metal electrode. This leads to a fear that the carbon material layer may come into direct contact with the metal electrode through the cracked or separated portion of the dielectric layer, resulting in microshorts. However, in the above approach wherein the carbon material layer is overlaid on the dielectric layer by electrophoresis using the metal electrode formed with the dielectric layer as the anode, even if the dielectric layer is cracked or separated from the metal electrode, the metal electrode is re-anodized at a portion corresponding to the cracked or separated portion of the dielectric layer so as to form the dielectric layer thereat. Thus, the microshorts between the metal electrode and the carbon material layer are obviated.
In the solid electrolytic capacitor according to the invention, a usable carbon material is at least one selected from the group consisting of natural graphite, synthetic graphite, coke, carbon nanotube, acetylene black and metal-doped fullerene. The metal-doped fullerene may include an alkali metal.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.